This invention relates to polyvinylidene fluoride resin filaments. In the following and throughout the present specification, the latter kind of resin will be referred to briefly as PVDF.
It is commonly known that conventional PVDF-filaments represent highly superior knot strength over other kinds of resin filaments, thus the former being highly adapted for use as fishing lines or the like. Among other physical properties, however, these filaments, when used for fishing lines and the like, should desirably have a high knot strength when subjected to an impact load. Such knot strength as above will be referred to briefly as the impact knot strength hereinafter throughout the specification and claims. It should be stressed that the impact knot strength does not always bear an intimate relationship with the knot strength per se.
A preferred conventional process for the manufacture of PVDF-filaments is such that melt-spun filaments are subjected to a first and a second stretching step at temperatures higher than 80.degree. C. for the purpose of stretching and thermal setting (refer to Japanese patent publication Sho No.--43-13399).
According to a further known process, melt-spun filaments are subjected to a stretching amounting to 2.5-10 times lengths for the realization of an optimal stretch and molecular orientation (refer to Japanese patent publication Sho No.--44-5359). According to still a further known proposal, melt-spun filaments are subjected to a primary stretching at a stretching factor defined by and between the primary and secondary inflection points appearing on the relative curve plotted by the double refraction factor. .DELTA.n or Young's modulus and the primary stretch factor, then thermally set and finally subjected to a secondary stretching (refer to Japanese Patent Publication Sho No.--53-22574).
In any of these known processes, the PVDF-resin not yet shaped should preferably be polymerized at a temperature lower than 60.degree. Cm still preferably lower than the critical temperature (30.5.degree. CO of polyvinylidene fluoride and the inherent viscosity (intrinsic viscosity).eta..sub.inh should be higher than 0.7 dl/g, when measured under such conditions that the solvent is dimethylformamide and the concentration is set to 0.4 g/dl. The upper limit is claimed in the second known technique disclosed in Japanese Patent Publication Sho No.--44-5359, to be less than 1.30 dl/g in terms of .eta..sub.inh, and in consideration of the spinnability of the PVDF. In the similar way, the value of .eta..sub.inh should be less than 1.50 dl/g in the case of the first known technique disclosed in Japanese Patent Publication Sho No.--43-13399. It will be clear from the foregoing that the melt spinning of PVDF having higher values of .eta..sub.inh than 1.30, especially than 1.50 dl/g, can be realized only with much difficulty in practice.